The present invention relates generally to solid state infrared radiation detectors, and more particularly, to an improved Platinum Silicide infrared diode. Note that this diode is used as a photodetector, so the terms "diode" and "detector" are sometimes used interchangeably in the description that follows.
All high performance infrared imagers require the detecting devices to be cooled. The longer the cutoff wavelength of the device, the more cooling is required. Doped silicon devices, Si:X, operate out beyond 20 micrometers in the very long wave infrared, but they require cooling to less than 13.degree. K. Mercury Cadmium Telluride detectors have the promise of working over the spectral range of 2 to 20 micrometers depending upon the fraction of mercury in the structure. Theoretically, they should operate at relatively high temperatures. Another detector material which has shown promise in the 3.0 to 5.0 micrometer band is Indium Antimonide (InSb). Theoretical calculations show that it should have cooling requirements in the 110.degree. K. range.
Unfortunately, these materials are compound semiconductors and are extremely difficult to fabricate. The individual detectors have been formed in the laboratory with the characteristics mentioned. However, in a real infrared system, large one or two dimensional arrays of diodes must be fabricated. Whenever large arrays are required of compound semiconductors, the arrays do not have the same characteristics as the individual diodes. This occurs because of the high degree of surface and bulk imperfections found in the compound semiconductor substrates. The operating temperatures which are predicted are never realized in finished arrays. Both InSb and HgCdTe usually require large arrays to be cooled to 77.degree. K. for proper low noise operation, regardless of theoretical calculations.
Schottky barrier diodes made from platinum silicide (PtSi) layers on silicon substrates are extremely promising for large two dimensional infrared focal planes. These devices are formed on industry standard silicon substrates of &lt;100&gt; orientation and have outstanding imaging characteristics in the 3.0 to 5.0 micrometer infrared atmospheric window. Unfortunately, these detectors also require cooling to 77.degree. K. for proper operation and suppression of thermal currents.
An infrared system which must be operated in space requires high reliability and as few moving parts as possible. The usual cooling method for infrared arrays in a terrestrial environment is a closed cycle refrigerator with many, moving parts. In space applications, system designers prefer to use passive cooling, which allows no moving parts. It works by radiating heat out to deep space. The minimum temperature achievable with this method of operation is about 105.degree. K. In order for an infrared imaging system to use passive cooling it must operate above this temperature.
The infrared devices described so far do not have the ability to operate at this temperature because of various technical problems. The task of providing improved infrared diode detectors is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,536,658 issued to Charlotte Ludington;
U.S. Pat. No. 4,533,933 issued to Paul Pellegrini et al;
U.S. Pat. No. 4,531,055 issued to Freeman Shepherd et al;
U.S. Pat. No. 3,864,722 issued to Carnes;
U.S. Pat. No. 4,346,291 issued to Chapel; and
U.S. Pat. No. 4,620,231 issued to Kosonocky.
The above-cited Ludington, Pellegrini and Shepherd references all describe infrared detector systems.
The patent to Carnes describes radiation sensing arrays which applies layers of n-type silicon and p-type silicon over the Schottky barrier layer. The patent to Chapel teaches a thermally isolated monolithic die using a layer of &lt;100&gt; crystal material of silicon dioxide (SiO.sub.2) wherein platinum silicide (PtSi) windows are formed in a protective layer. The patent to Kosonocky teaches a charge coupled diode (CCD) which employs a (PtSi) layer over line p-type silicon substrate.
The need remains to provide an IR detector which is able to operate at 105.degree. K. (or above) such that it requires only passive cooling. The present invention will allow the use of platinum silicide infrared detectors in infrared systems which are required to use passive cooling techniques. Previous technology has used PtSi infrared arrays fabricated on semiconductor grade silicon substrates of 100 crystallographic orientation. The new detectors must use a different silicon orientation, but in so doing they will allow detector operation at temperatures in the range of 110.degree. K. At this temperature, passive cooling in space can be accomplished.